1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to decreasing a spin up time for a disk drive by adjusting a duty cycle of a spindle motor PWM signal to maintain constant average input current.
2. Description of the Prior Art
The spindle motor of a disk drive is typically driven by a supply voltage provided by a host computer. The supply voltage is periodically applied to the windings of the spindle motor in a desired commutation sequence resulting in a rotational torque. During each commutation sequence, a selected winding is connected to the supply voltage through a pulse width modulated (PWM) control signal. A servo controller generates the PWM control signal having an adjustable duty cycle for controlling the desired torque of the spindle motor.
The disk drive must be designed to meet the current limitations of the supply voltage established by the vendor of the host computer, including the circuitry that drives the spindle motor. This is of particular concern during the start up process which requires more torque to spin the disk up to an operating speed. The prior art has suggested to monitor the peak current passing through the windings of the spindle motor in a current controlled feedback loop for controlling the speed of the spindle motor during spin up as well as during the normal, steady state operating mode of the disk drive. The prior art has also suggested to monitor the peak current in the windings of the spindle motor to current limit and thereby protect the driver circuitry. However, to ensure the supply voltage current limitations are not exceeded prior art disk drives are typically designed so that during the spin up process the spindle motor draws as little as 60% of the available current from the supply voltage. This increases the spin up time which is undesirable, particularly as the operating spin rate increases. The host computer typically imposes a constraint on the maximum spin up time before a time-out error is generated. Further, the increased spin up time increases head wear in disk drives that park the head on a landing zone of the disk.
U.S. Pat. No. 5,442,272 discloses to charge a capacitor prior to spinning un the disk and to discharge the current stored in the capacitor into the spindle motor windings during the spin up process in order to limit the amount of current drawn from the supply voltage. The capacitor must be large enough to store a sufficient amount of current to enable higher torque and faster spin up times. However, mounting a large, discrete capacitor on the printed circuit board assembly (PCBA) is undesirable. In addition, there is an undesirable delay associated with charging the capacitor prior to initiating the spin up process.
There is, therefore, a need to improve the spin up process for disk drives without exceeding the current limitations of the supply voltage.